From: Paul E. McKenney paulmck@kernel.org
commit 10c535787436d62ea28156a4b91365fd89b5a432 upstream.
Currently rcu_preempt_deferred_qs_irqrestore() releases rnp->boost_mtx before reporting the expedited quiescent state. Under heavy real-time load, this can result in this function being preempted before the quiescent state is reported, which can in turn prevent the expedited grace period from completing. Tim Murray reports that the resulting expedited grace periods can take hundreds of milliseconds and even more than one second, when they should normally complete in less than a millisecond.
This was fine given that there were no particular response-time constraints for synchronize_rcu_expedited(), as it was designed for throughput rather than latency. However, some users now need sub-100-millisecond response-time constratints.
This patch therefore follows Neeraj's suggestion (seconded by Tim and by Uladzislau Rezki) of simply reversing the two operations.
Reported-by: Tim Murray timmurray@google.com Reported-by: Joel Fernandes joelaf@google.com Reported-by: Neeraj Upadhyay quic_neeraju@quicinc.com Reviewed-by: Neeraj Upadhyay quic_neeraju@quicinc.com Reviewed-by: Uladzislau Rezki (Sony) urezki@gmail.com Tested-by: Tim Murray timmurray@google.com Cc: Todd Kjos tkjos@google.com Cc: Sandeep Patil sspatil@google.com Cc: stable@vger.kernel.org # 5.4.x Signed-off-by: Paul E. McKenney paulmck@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/rcu/tree_plugin.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
--- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -554,16 +554,16 @@ rcu_preempt_deferred_qs_irqrestore(struc raw_spin_unlock_irqrestore_rcu_node(rnp, flags); }
- /* Unboost if we were boosted. */ - if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex) - rt_mutex_futex_unlock(&rnp->boost_mtx.rtmutex); - /* * If this was the last task on the expedited lists, * then we need to report up the rcu_node hierarchy. */ if (!empty_exp && empty_exp_now) rcu_report_exp_rnp(rnp, true); + + /* Unboost if we were boosted. */ + if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex) + rt_mutex_futex_unlock(&rnp->boost_mtx.rtmutex); } else { local_irq_restore(flags); }